This proposal focuses on the role of the cell cycle inhibitor p57KIP2, on the differentiation of ventricular myocytes. Previous studies suggest that p57KIP2 plays an important role regulating the balance between proliferation and differentiation in the developing heart and suggests its involvement in causing a thin-walled ventricular myocardium phenotype (dilated cardiomyopathy) in the mouse. We hypothesize that cardiac over-expression of p57KIP2 depletes the ventricular proliferative zone and leads to a thin myocardium phenotype by causing early terminal differentiation of cardiomyocytes. The goal of this proposal is to test this hypothesis by examining the effects of p57 KIP2 over-expression in the mouse and zebrafish animal models. I will pursue two specific aims: 1. Examine the role of p57KIP2 in the generation of the thin-walled myocardium phenotype in the mouse. - First, I will analyze the pattern of expression of p57 KIP2 in established murine models of thin myocardium. -Second, I will examine the effects of p57 KIP2 over-expression in the mouse heart by inducing Cre-loxP mediated activation driven by the myosin light chain-2 ventricular (MLC-2v) promoter. 2. Identify the zebrafish p57 KIP2 homologue, isolate its full length cDNA and perform in depth analysis of this zebrafish homologue, including: -detailed analysis of its temporal and spatial expression pattern by whole mount in situ hybridization and RT-PCR. -study the effects of the constitutive and cardiac specific over-expression of p57 KIP2 in the zebrafish. -study the effects of the morpholino induced inactivation of p57KIP2 in the zebrafish. These experiments will form the foundation for further investigating the role of p57KIP2 in the settings of dilated cardiomyopathy, ventricular hypertrophy and cardiac regeneration. Understanding the mechanisms underlying withdrawal of cardiomyocytes from the cell cycle will be important for the treatment of a wide range of cardiovascular diseases.